Just when I thought I had it sussed, ie down bag inside synthetic, an unrelated trip to the loft threw a spanner in the works. There, tucked away at the side of the gear pile, was a suspiciously fluffy-looking storage bag.
In the bag was a Snugpak Sleeper Lite, rated IIRC, to about -10C. I'd completely forgotten I had it. From memory, it is very warm but, like all synthetics, relatively bulky. I'll have a go it stuffing it into things over the next day or 2. I think I'll still go with the down/synth' combo' but this one might work out as an alternative to the Alpkit for The Boy.Interesting.